A variety of multimedia services are widely coming into practical use with increasingly faster networks, rapid widespread use of personal computers and digital television receivers or the like among general households in recent years. Above all, digital broadcasting systems and content delivery systems using the Internet or the like generally compress and encode content data such as video and sound based on standards such as an MPEG (Moving Picture Experts Group) system, use fractions of encoded data, which are compressed and encoded content data, as packets, transmit a set of packets as a stream signal or record the encoded data into a hard disk, DVD or memory card or the like. Furthermore, information devices such as a personal computer, set top box, portable information terminal apparatus or mobile phone that receive such a stream signal extract encoded data included in the stream signal, perform decoding on the extracted encoded data and thereby reproduce the content data.
Among such image compressing and encoding systems, there is a JPEG (Joint Photographic Experts Group) system, which is widely used for digital cameras or the like as a system for compressing and encoding still images. On the other hand, as a system for compressing and encoding video images, there is an MPEG-1 system for recording moving images into a CD-ROM or the like. In this way, basic techniques such as an image compression/encoding technique based on, for example, discrete cosine transform have been established. Based on such basic techniques, for example, an MPEG-2 system used, for example, for digital broadcasting, MPEG-4 system used for content using the Internet and MPEG-4AVC system intended for higher compression/encoding are used as standard systems in the fields handling image signals.
The basic algorithm of such image compression/encoding is a hybrid compression/encoding system that combines a motion compensation prediction system with the above described discrete cosine transform system. The hybrid compression/encoding system performs image compression/encoding on an image by combining both data obtained by applying discrete cosine transform to data itself within one picture and data obtained by applying discrete cosine transform to motion compensation inter-picture prediction data.
Furthermore, discrete cosine transform is performed in units of a block called “macro block.” Each pixel data making up this macro block is transformed into a plurality of coefficient values indicating amplitude magnitude per frequency from a DC component to a high-frequency component through discrete cosine transform. Furthermore, each coefficient value is quantized using a technique based on each compression/encoding system. Images are compressed/encoded based on such an algorithm. Furthermore, images can be reconstructed by performing decoding in association with compression/encoding on the data generated by compression/encoding. Specifically, pixel data can be reconstructed in macro block units by performing inverse quantization on quantized coefficient data, which is a quantized coefficient value, and applying inverse discrete cosine transform to the data reconstructed through inverse quantization.
On the other hand, since various image compression/encoding systems are proposed in this way, there are conventional proposals of apparatuses provided with a circuit for compression/encoding so as to support different compression/encoding systems in one apparatus such as a personal computer and portable information terminal apparatus. For example, Patent Document 1 proposes a technique of an image decoder provided with a H.263 system inverse quantization unit and an MPEG-2 system inverse quantization unit so as to perform inverse quantization by switching between the respective inverse quantization units according to quantized coefficient data based on the respective encoding systems.
However, individually configuring the respective inverse quantization units based on the respective encoding systems as in the case of the conventional image decoder causes the problems that: the circuit scale increases; and if a new encoding system is proposed, for example, it is difficult to speedily respond to the proposal and an LSI (large scale integrated circuit) or the like for performing decoding needs to be redesigned.    Patent Document 1: Japanese Patent Laid-Open No. 2004-201345